I believe the whole idea of monogamy stems from patriarchal properties rights that were upheld by "divine providence" so I can't agree that as an atheist I must or any other atheist must engage in sex with only their significant other.

The roots of a rule do not necessarily validate or invalidate a rule, so we can ignore where the tradition of monogamy came from (by the same token, you can't invalidate the U.S. Constitution by saying that it's the work of sexist slave-owners). I agree that there is no moral reason why an atheist must be monogamous. However, an atheist must, as a moral individual, uphold social contracts, so no fair fooling around on your spouse and then trying to use this argument to justify yourself.